1. Field of the Invention
This invention relates to active retrodirective antenna arrays and more particularly to an active retrodirective antenna array (ARAA) that uses an optical carrier beam to reflect an RF pilot signal off a target to create a virtual beacon to individual nodes in the array.
2. Description of the Related Art
For many applications, it is necessary or desirable to be able to automatically track a received signal. In a radar or communication application, for example, it might be useful to be able to track a source of a received signal. A satellite or another RF signal, for example, might be tracked and a return signal automatically transmitted back to the satellite or RF signal source in response thereto. An automatic tracking system might allow for more effective and efficient direction of a return beam to complete an RF signal source or communication link.
Conventionally, beam tracking has involved physically pointing an antenna in the direction of the received beam or processing the received signal with a tracking algorithm to steer a phased array antenna in the direction of the received signal. At any given time, the phase shifts applied to each array element in a phased array determine the direction of the main beam for both reception and transmission. Considerable analog and digital processing is required to track a target and radiate a beam in the direction of said target. Thus, while phased array antennas have been used for some time, automatic beam steering in response to an RF signal with a phased array antenna has been somewhat problematic to date.
An active retrodirective antenna array (ARAA) is a phased antenna array that automatically steers its transmitted beam towards the apparent source of an incoming pilot signal. The modifier “active” means that the radiated power is generated by sources associated with the antenna, rather than by reflection of an incident signal as in a passive retrodirective antenna (e.g., corner reflector). Such arrays, which are also known as “self-focusing” arrays, have been suggested for some time. In such arrays, the transmitted wavefront duplicates the incoming pilot signal wavefront whatever its shape. The self-focusing property is important because it means that the transmitted power is focused back on the pilot source whatever the state of the intervening propagation medium, provided that the state persists for the round-trip propagation time. The retrodirective properties of proposed ARAA's is achieved by “conjugating” a pilot signal incident at each array node. Though not yet in widespread use, ARAA's are expected to become an important part of phased array technology. They have, for example, been proposed for microwave power-transmission from orbiting solar power stations, communication satellite transmitting arrays, and aircraft transponders.
When a retrodirective array receives an RF pilot signal from an unspecified direction, each array node receiving the RF pilot signal automatically generates and transmits a phase-conjugated RF pilot signal in the reverse direction without any previous knowledge as to the location of the signal source and without the need for sophisticated digital signal processing. The nodes may be randomly dispersed over a wide area, and knowledge of their positions is not required. The source of the RF pilot signal is a microwave beacon attached to a cooperative target. The beacon broadcasts the RF pilot signal which is received by some or all of the nodes of the array. The phase of the received signal is conjugated with a phase reference derived from the received pilot signal itself, amplified, and retransmitted at a different frequency to preserve isolation. The process of conjugating the phase of the received pilot signal causes the phase accumulated during propagation from the beacon to each node to be cancelled during the return trip. As a result, the signals transmitted by the individual nodes converge at the beacon with nearly identical phases, causing them to add coherently. U.S. Pat. Nos. 4,148,031 and 6,630,905, which are hereby incorporated by reference, describe different techniques for processing the received pilot signal to extract a phase reference for performing the phase-conjugation.